North Seattle Community College's
PHYSICAL GEOLOGY 101
Instructor:  Tom Braziunas

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Geologic Map Quadrangle
Clark County, Nevada  

GEOLOGIC MAP EXERCISE

@2002 -- The information contained in this document is copyrighted.
No reproduction may be made without prior approval from the author (Dr. Tom Braziunas).

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I. An Introduction

Geologic maps are topographic maps with information on geology colorfully added to them. Colors, labels and symbols indicate particular rock "units" of the underlying geology.  Each unit (or "formation") consists of rocks that share a common composition and texture and therefore are identified as a "unit."  Sedimentary rock layers (also called "strata") can be assigned names that conveniently identify them wherever they are exposed without the need to know the details of their ages or origin.

Once a formation has been dated, we know its relative place on the geologic time chart (page 374 of our textbook) and its approximate absolute age in millions of years.  The rock units themselves may not have been dated directly, but they may be correlated with other rocks elsewhere whose ages have been determined through relative and radiometric dating.

At any one location, the rocks and sediments may be overlain by vegetation and hidden.  The geologic map shows the rock units that would be exposed at the surface if the vegetation were removed.  The spots where rocks are actually exposed are called "outcrops" or "rock exposures".   Geologists study rock exposures and infer the geology found in the areas where the bedrock is covered up.  In this way, they create geologic maps. 

We will practice dealing with geologic information by using the geologic map of the Grand Canyon.  Most of the rocks in the Grand Canyon are exposed at the surface and therefore are easier to observe and relate to a geologic map.  We have 25 questions to answer along the "trail," each worth one point.  Use the "Week 7 -- Lab Homework Part 2" form to submit your answers.  TOTAL POINTS POSSIBLE = 25.

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II. A Review of Topographic Maps:

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Let's revisit the Grand Canyon topographic map from an earlier lab exercise so as to review some basic principles before incorporating geology into the picture.  One of the Grand Canyon's grand features is a natural prominence called the Isis Temple.  Click on the first thumbnailed image below and study the topography.  Remember that contour lines are used to show elevation and shape of the terrain.  A contour line connects all points of equal elevation.  The map also represents about 2 miles of horizontal distance across (west to east).

Question 1:  If you walked from the the 4000 foot contour line up to the top of the Isis Temple, what would be your gain in elevation?   ___________

Question 2:  What is the contour interval on this map and how far apart are the "index contours" (the marked contours as explained in the topographic map lab)?   ______________  and ____________

Question 3:  Between which two "index contours" will you encounter the steepest slope on your climb (that is, which two index contour lines are generally the closest together on this map)? ________________

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Try to picture Isis Temple in your mind based on the contour lines.  Then compare your image with an actual photograph of Isis Temple by clicking here.  

Question 4:  Please describe where the steep slope is in the photograph.  Where is the steepest cliff located with respect to the top and bottom of the photograph?   Does it fit where what we determined from the contour lines?
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Now we are ready to add in the geologic information we have on the Isis Temple.

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III. Geologic Maps:

Click on the thumbnail of the geologic map of the Isis Temple below and compare it to the topographic map of the Isis Temple above.  Notice that the maps are nearly identical except that information on the geologic rock units has been incorporated using colors and labels.  The colors do not have any significance with respect to the type of rock unit -- color is simply used to differentiate different rock formations (layers).

Question 5:  What color and two-letter label are associated with the rock formation that produces that steepest slope?  Is it pink, tan, purple, green, or .... ?  Is it Ph, Pc, PPs, or .... ?   ______________

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An "Explanation" showing the colors and symbols for the various rock types accompanies each geologic map.  This "legend" tells us how to read the geologic information on the map.  The legend for the Isis Temple geologic map is the thumbnail image below (I scanned it in three parts).  Please click on it and note how the formations (distinctive rock units) are presented and how their geologic time periods are denoted.  

Usually the older (more ancient) rocks are shown at the bottom of the "Explanation" and the descriptions of rock types gets progressively younger as you go up the column of rock descriptions.

 

Question 6:  What is the formation name for the rocks that produce that steepest slope on the Isis Temple? (Kaibab Formation, Toroweap Formation, or ..... ) _____________

Question 7:  What geologic time period does it belong to? (Permian, Mississippian, Cambrian, or ..... )  _________________

Question 8:  About how old is this rock unit based on the absolute ages shown on the geologic time chart on page 374 of your textbook?    Match up the geologic time period you determined above with its age on this chart.  So the absolute age of this formation is between ____________ and __________ million years old.

Question 9:  What is the full name of the formation that is exposed on the Isis Temple above the one we have just worked on? ________________

Question 10:  The rocks that form the steepest slopes are the ones which are harder to erode.  Let's compare the formation we just named in Question 9 with the one named in Question 6.  Based on the closeness of their contour lines in general, which of these two units is hardest to erode? ______________

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III. Regional Geology:

The geological map of the Isis Temple is part of a much larger geologic map of the Grand Canyon region.  The image below shows much more of the geology of the Grand Canyon and allows us to draw some conclusions on a larger scale about the history of the Grand Canyon.  The map represents about 4 miles of horizontal distance across (west to east).

Click on the image and be sure to find Isis Temple again.   Find the Colorado River and the Kaibab Plateau.  Hopefully we are picturing the Grand Canyon's topography.  Are we picturing its geology too?  If rock layers have been tilted and/or folded, each rock formation will be exposed at different elevations at different locations just like books that have tipped sideways on the bookshelf.   However, if the layers of rock in a region are flat (horizontal) with little tilting or folding -- like a layer cake -- then the same rock unit will be exposed at the same elevation everywhere.  Notice whether or not most of boundaries of the different colors on the map follow the contour lines. 

Question 11: Are the higher-elevation formations of the Grand Canyon more like a layer cake or a tipsy bookshelf? __________________

Question 12:  The Colorado River which carves out the Grand Canyon is the lowest elevation feature.  What is its approximate elevation? (It is difficult to determine this but take your best educated guess by looking at the contour lines next to the river!)   _____________

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Question 13: The Kaibab Plateau is at the top of the Grand Canyon (the light blue formation).  What is its approximate elevation? _____________

Question 14: What is the difference in elevation between the top of the Grand Canyon and the Colorado River at the bottom? __________

Question 15:  What rock unit formation is found at the top of the Grand Canyon? ______________

Question 16:  What rock unit formation is generally found at the bottom (next to the Colorado River)? ________________

Question 17:  What is the approximate difference in absolute age between the rocks at the top of the Grand Canyon and the rocks exposed at the very bottom on the Colorado River? (Find the geologic time periods of the formations and convert them to absolute numbers using the chart on page 374 in order to calculate this.  Explain the steps you took.) _____________

Question 18: Do the rock layers of the Grand Canyon follow the Law of Superposition?  Explain. ____________

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IV. Reconstructing Geologic History:

Compare the geologic map above with a photograph (click here) of the Grand Canyon showing the same features as the map.  The Kaibab Plateau is the distant plateau in the background.  Remember that this is the Kaibab Formation (the light blue color on the geologic map).

Question 19:  Notice the conspicuous white cliff-forming unit way in the background.  Use the geologic map to determine what unit this is.  (Remember that it must be a unit with contour lines very close together!) ______________________

Let's say we can walk right up to this white cliff and examine it more closely.  We see (1) cross-bedding in many directions; (2) well-rounded, well-sorted quartz sand grains, and (3) the tracks of terrestrial reptiles.  

Question 20:  What sedimentary environment does this likely represent and why? (Look again at Chapter 6.) ______________________________

Now let's walk down to the other steep cliff, the one we focused on at the beginning of this exercise.  Can we see this "Great Red Wall" (400 feet in height) in the photograph?  When we look at it closely, we see that it contains an incomprehensibly vast collection of fossilized skeletons of fish, trilobites, corals, and brachiopods.  Wow!

Question 21: What type of environment was this formed in? _____________________

Question 22:  What might have happened to this area between this time (earlier in the Grand Canyon's history) and the time of the white cliff above (later in the Grand Canyon's history)? ______________

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V. Geologic Cross Sections:

Geologic cross-sections are like topographic profiles (review the one which we drew for the topographic map lab).   They show "side views" of the topography along with the inferred underground geology.  For example, the geologic cross-section below (in two parts) shows a slice of the topography along a straight line (a "transect") running northeast to southwest across the canyon. 

Look for this line on the geologic map above -- it is a diagonal line that cuts across the Cheops Pyramid.  It is along this line that the cross-sections below have been drawn.

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Notice (in piece B of the cross-section) the "Great Angular Unconformity".    This is not labelled -- but see how the rocks at the bottom of the profile have been tilted while the younger rocks on top are horizontal.    The earlier sequence of rock formations were tilted before the more recent sequence of rock formations was deposited on top of it.

Question 23:  Pinpoint the time at which this tilting took place.  Find the youngest layer which has been tilted and the oldest layer which has NOT been tilted.  The tilting must have occurred between these two times.  Between which two rock formations did it occur? ___________

There is also a major Disconformity (look up the definition of this type of unconformity in Chapter 8!) in addition to the "Great Angular Unconformity").  This disconformity occurs between two horizontal rock formations and represents a major gap in time during which any sediments deposited were subsequently eroded away.  You cannot tell when this occurs by looking at the geologic cross-sections (the layers above and below this gap in time are horizontal) but you can determine where there is a major gap in time by looking at the map legend from earlier in this exercise.  Look for the missing geologic periods in the Paleozoic Era as compared to the geologic chart on page 374.

Question 24:  What time periods are missing from the middle of the Grand Canyon geology? ____________

Finally, we notice (on piece A of the cross-section especially) that many faults have occurred in the area of the Cheops Pyramid.  These are the dark lines at an angle that slice through the rock formations.  When did all this faulting take place?   We can tell using the Principle of Cross-Cutting Relationships (Which layers do the faults cut through and which do they NOT cut through?).

Question 25:  Between the formation of what two rock units did this faulting take place? ___________

I hope we had some fun on this trip along with the very hard work!

 

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